This invention relates generally to signal level control circuits and, more particularly, to variable gain amplifers that utilize digital circuitry for changing the gain to effect automatic signal level control.
In most all of the commonly employed communication systems or measurement systems, signal level control circuits are commonly used at some point in the system to maintain a more or less constant signal level output. Natural phenomena, randomly occurring, or unknown conditions all can produce undesirable variations in the level of a signal. These level variations must usually be substantially minimized by some type of gain control circuit before the signal can be effectively utilized to provide reliable operation.
Most conventional gain control circuits are of the analog variety. Such circuits usually must employ a nonlinear element to control amplifier gain, such as a diode or field effect transistor, to make full use of the dynamic range in gain available from a given amplifier. Unfortunately, the nonlinear element also introduces harmonic distortion into the signal whose level is being maintained constant at the output of the gain control circuit. The operation of these analog circuits also makes it difficult to provide an accurate indication of the level of signal being applied to the gain control circuit from within the circuit because the response to changes in level of the input signal produced nonlinear variations in the control signal applied to the nonlinear device which is the source of the input level information.
Recently, digital gain control circuits have come into being which offer advantages over their analog equivalents. In digital gain control circuits the circuit elements which are used to effect gain control are linear and do not serve as a source of distortion. Digital control circuits are clocked and can be readily synchronized with timing operations of other control circuits to perform operations within predetermined intervals, thereby enhancing the performance of the system. A principal drawback common to both analog and digital gain control circuits is that unavoidable and erratic high noise levels, which are often present in the signal, deleteriously effect the stability of gain control circuits and disrupt their operation.
An object of this invention is to improve substantially the stability of automatic gain control circuits while operating in a high noise ambient by providing immunity thereto.
Another object of the invention is to produce digital signals indicative of the level of the signal applied to automatic gain control circuits by the response of same to the level of signal.